I have a Hydor Theo in my 2.5. It really doesn't matter how many watts you get, you can go for more than it needs but make sure it will fit in your tank. What is really important is that your heater is adjustable and that you get a thermometer to monitor the temparature.
On this website, they tell you the lenght of the heater: http://www.drsfostersmith.com/produc...m?pcatid=11368